Coal is the most abundant natural energy source in the world. A significant portion of the U.S. domestic energy requirements are met by burning coal as a fossil fuel. There are various types of coal found within the U.S., i.e., anthracite, semi-anthracite, low-volatile bituminous coal, medium and high volatile bituminous coal, sub-bituminous coal, and lignite. Coals such as anthracite and semi-anthracite typically have high ash and sulfur contents and therefore require beneficiation prior to use.
The primary purpose of coal beneficiation is to reduce the incombustible ash content thus enhancing the heat content. Reduction in the ash content results in savings in transportation and ash disposal costs. Sulfur, mainly in the form of purite, is also reduced.
Another important economic factor to be considered in coal processing is the recovery and reuse of process water. Water is typically very expensive and there are often limits on total usage. Also, strict environmental controls prohibit or severely limit discharge of process water. Thus, it is imperative that solids be efficiently removed from the process water and water recycled to the process stream.
Beneficiation of coal is effected using two primary properties of coal, i.e., (1) differences in specific gravity between coal and its impurities, and (2) differences in surface characteristics between coal and its impurities. Since the higher ash content fractions are usually found in the finer coal sizes, some plants only screen out these sizes to beneficiate the coal. However, since the quantity of such fine coal is on the rise, even this is treated.
A coal beneficiation plant may be broadly categorized into specific gravity separation and fine coal treatment. In gravity separation, cleaning units make use of the differences in specific gravity between coal and its impurities to effect separation. Normally, the specific gravity of the clean coal is less than its impurities. Some commonly used equipment for gravity separation are: jigs, heavy medium baths and cyclones, washing tables, water only cyclones and spirals.
Fine coal treatment incorporates a flotation cell(s), clean coal filter and thickener. In the flotation cell, a collector and frother are added to the flotation feed. The collector (e.g., diesel oil #2) selectively imparts hydrophobicity to the coal particles. This increased hydrophobicity makes the air bubbles more likely to attach to the coal particles. The frother (e.g., an alcohol based product) reduces the surface tension of the air/water interface, thus making a stable froth.
The concentrate (i.e., clean coal) from the flotation cells goes to the clean coal filter and is dewatered. The tailings from the flotation cell go to the thickener where they are thickened and discharged.
The thickener is treated with coagulants and flocculants to enhance settling. Typically, the coagulants and flocculants are added at several points along the feed line to the thickener and in different sequences. Coagulation is the destabilization by surface charge neutralization of stable negatively charged particles that are in suspension (i.e., settleable or dispersed) through the utilization of inorganic salts or cationic polyelectrolytes. Flocculation is the aggregation of finely divided particles which are suspended in a liquid through the utilization of an entrapping agent (i.e., an inorganic flocculant) or a bonding agent (i.e., an organic flocculant) that brings the particles together.
Although some inorganics, principally alum and iron salts, are still used as coagulants, water soluble organic polymers are more commonly used as coagulants. Both naturally occurring and synthetic polyphers find use as coagulants and flocculants in the mining industry. The principal natural polymers used are starch and guar, both of which are high-molecular weight polymers of simple sugars (i.e., polysaccharides). Starch is a polymer of glucose consisting of a mixture of linear (amylose) and branched segments (amylopectin).
Synthetic polymers have the advantage that they can be tailored to a specific application. This has resulted in a wide range of commercially available coagulants and flocculants of varying charge, composition, and molecular weight. The most widely used synthetic coagulants are polydiallyldimethylammonium chloride (poly-DADMAC or DADMAC) and condensation polymers of dimethylamine and epichlorohydrin (Epi/DMA). These structures vary greatly in molecular weight and are in the range of 20,000 to 100,000.
The present inventors have developed various novel hydrophobic polyelectrolyte copolymers which may be used as coagulants in the thickening process during coal beneficiation. These hydrophobic monomers exhibit improved performance or activity in coal tailings thickening than do conventional inorganic and organic coagulants. In addition to the hydrophobicity, the hydrophobic monomer used to synthesize the polyelectrolyte copolymers according to the present invention produce copolymers with substantially higher molecular weights than conventional synthetic DADMAC homopolymers prepared under the same conditions. Furthermore, the incorporation of a quaternary group, such as benzyl, into the copolymer of the present invention reduces the bulk viscosity of the resultant copolymer relative homopolymers of DADMAC with comparable molecular weights. As such, the hydrophobic polyelectrolyte copolymers of the present invention exhibit higher polymer concentrations than conventional organic coagulants. These hydrophobically associating copolymers also demonstrate enhanced performance with replacement ratios on the order of 0.45-0.50.
The present invention also provides many additional advantages which shall become apparent as described below.